Alarm systems for installation in movable vehicles such as automobiles are well known. These systems typically include an alarm controller that is coupled to an alarm annunciator and sensors located throughout the vehicle for detecting specific events indicative of a security violation. The alarm controller typically includes a microprocessor or microcontroller with sufficient memory and input/output (I/O) interface devices to effectively couple the controller to the sensors. The system is typically powered from the battery source from the vehicle. Sometimes a back-up battery is included for the alarm controller so the controller remains powered in the event an intruder cuts the battery cable.
A variety of sensors are used in such alarm systems. These sensors include current sensors that are coupled to the interior lights of the vehicle to detect the current flow for operating a light when a door is opened; a motion detector to detect movement in a radiated field about the vehicle; and a shock sensor to detect impact to the vehicle usually indicative of an unauthorized entry event or vandalism. The signals from the sensors are presented to the alarm controller and the alarm controller typically evaluates whether the sensor signal requires an alarm, or no response for the alarm annunciator.
One problem with these previously known systems is the difficulty in evaluating the severity of the violation event. The vehicles are often parked in areas, such as parking garages, shopping mall parking lots, and the like, where there is significant pedestrian traffic and/or incidental bumping of the vehicle. This non-destructive contact with the vehicle or radiated field about the vehicle is difficult to distinguish from violation events. An alarm system that can distinguish between incidental bumping or casual traffic and the presence of an intruder generates fewer false alarms. False alarms are undesirable because they drain the vehicle battery, and, if too frequent, may be ignored by security personnel.
Previously known systems have incorporated a scheme of counting the number of events detected by a sensor to determine whether the sensor is detecting an intrusion or vandal event. In response to a count below a predetermined alarm number, the system generates a warning while an alarm is generated for count values exceeding the alarm number. One problem with this counting scheme is that in an area of high traffic or significant shock, such as the vibrations created by traffic in a multi-floor concrete garage, enough events still occur to exceed the alarm number and generate a false alarm. Thus, the counting scheme does not substantially reduce the chance of false alarms in all areas.
Another attempt to provide an alarm system with the ability to evaluate the severity of a sensor event is to provide a single threshold value for comparing the magnitude of a sensor signal to a predetermined threshold. If the magnitude of the signal from the sensor exceeds the threshold, an alarm is sounded, otherwise, no response is generated. One problem with the single threshold scheme is the inability of the alarm system to warn a tentative intruder that the vehicle is equipped with an alarm system. For example, an intruder may tap some portion of the car to see if an alarm is actuated and gaining a false sense of security when no alarm sounds, increase the impact of the blow to gain access to the car. While the intruder may flee in response to the alarm generation when the sensed blow magnitude exceeds the single threshold, significant damage may be done to the vehicle.
In an effort to remedy the single threshold problem, alarm systems have been developed that permit a user to adjust the threshold level for different situations. This is typically done by putting the alarm system in a test mode and then striking the vehicle with a force that is used by the alarm controller as the single threshold. While this gives the user some control over the sensitivity of the threshold setting, it does not provide both a warning and an alarm. What is need is an alarm system that provides both a warning and an alarm without the drawbacks of the known counting scheme.